In a machine with rotating components it is often desirable to measure the distance between a rotating component and nearby static components of the machine. This distance, referred to as the clearance, is necessarily kept larger than a minimum clearance requirement such that the rotating component does not strike the static component during operation of the machine. However, relatively large clearances in rotating machines are also undesirable as they can reduce the efficiency of the machine.
As an example, in a turbine engine such as a gas turbine engine the blades of the turbomachinery rotate at high rates of speed relative to a static casing or shroud that houses the rotor. The gas turbine engine may comprise a compressor, a combustor, and a turbine. From a cold start to operating temperatures the various rotating and static components may undergo thermal expansion. It is therefore desirable that even at full operating temperatures a sufficient clearance is maintained. A sufficient clearance ensures that a rotating rotor blade does not strike the casing. However, in the case of a turbine engine, clearances larger than that sufficient to prevent blade impingement allow a flowpath for combustion gases to bypass the turbomachinery such as compressor blades or turbine blades, thus reducing the efficiency of the turbine engine. The measurement of clearances is additionally beneficial when conducting performance diagnostics on turbomachinery.
Understanding then that clearances are critical to the safe and efficient operation of rotating machines, it is desirable to be able to measure them. Unfortunately, in many rotating machines, access to the space between a rotating component and static component is extremely restricted. In many machines, accessing this space requires removal of at least a portion of the casing or shroud around the rotating component, which is typically a difficult, time-consuming, and expensive process. Additionally, as noted above there can be significant changes in clearances depending on whether a machine is at cold or operating temperatures. As it is generally not practicable to directly measure a clearance in an operating machine, it is necessary to have a method of measurement during operations that does not imperil the safe operation of the machine.
Previous solutions to this problem include the use of rub pins. Rub pins are typically columnar shaped and formed from a soft metal or similar material which can be abraded when contacted by a rotating component during operation. A rub pin is affixed to a static component such that it is disposed in the space between a rotating component and the static component, and the machine is then started and brought to operating temperatures or pressures. Once the machine has been operated in a desired manner or for a desired length of time, the machine is secured and cooled down. The machine is then at least partially disassembled to allow for inspection and retrieval of the rub pin. Due to the impingement of a rotating blade on the pin, the pin will have worn down to indicate the clearance between the rotating component and static component. A measurement of the pin length, when measured between the worn end and the surface of the static component, is a measurement of the machine's clearance. Although effective at obtaining accurate clearance measurements, the use of rub pins is a time-consuming and expensive process because it requires securing and partially disassembling the machine to insert the pins, operating the machine, and then securing and partially disassembling the machine a second time to retrieve and inspect the pins.
Another solution is described in European Patent Application 2,236,977. This application discloses a rub pin having at least one embedded wire which completes a circuit through the pin. When the pin is installed in the static component and protrudes into the rotating component path, the wire will be broken by blade impingement at a certain clearance. Breaking the wire causes an interruption in the circuit, which can be used as an indication that the clearance is insufficient or has surpassed a predetermined measurement. This solution is thus effective at monitoring machine performance for a single clearance, but does not provide a means for measuring over a wide range of clearances and additionally requires electrical circuitry for effective use.
It is thus desired for an improvement in the art of measuring clearances which would enable such measurements to be taken in a safe and efficient manner.
The present application discloses one or more of the features recited in the appended claims and/or the following features which, alone or in any combination, may comprise patentable subject matter.